FIG. 1 shows a direct current changer with instantaneous value current limiting which is also known as pulse by pulse limiting and is known from DE-PS 26 13 896.
In the single-ended forward converter shown in FIG. 1, the input voltage U.sub.E is applied to the capacitor 1 and the output voltage U.sub.A to the capacitator 13. A series circuit formed from the primary winding 91 of the transformer 9, the drain-source junction of the field-effect transistor 6 and the measuring resistor 3 is connected in parallel with the capacitor 1. The Z diode 5 which serves to limit the voltage is connected in parallel with the series circuit comprising the measuring resistor 3 and the source-drain junction of the field-effect transistor 6. The rectifier diode 10 is connected between the secondary winding 92 of the transformer 9 and the capacitor 13. The freewheeling diode 1 is arranged in a shunt arm following the rectifier diode 10. The inductor 12 is located in a longitudinal branch between the freewheeling diode 11 and the capacitor 13.
The control electrode of the field-effect transistor 6 is connected to the control device 8 which contains a driver, a clock generator and a pulse-width modulator. The control arrangement 8 is controlled both by the comparator 4 and by the voltage controller 14. The comparator 4 is connected by its negative input via the reference voltage source 2 to one terminal of the measuring resistor 3 and via its positive input directly to the other terminal of the measuring resistor 3. The voltage controller 14 serves to control the output voltage and is therefore connected by its actual value input to the output of the converter.
With a circuit arrangement of this kind, adequate current limiting can be achieved in many cases. However, in the case of converters with a relatively high output voltage, for reasons based on the principle of a so-called "running-out of the characteristic" occurs; that is to say, the limiting effect is reduced or eliminated in the case of a short circuit. This can ultimately lead to the destruction of components. The cause for behavior of this kind lies in the remaining minimum pulse width of the switch-off and on pulse which is produced by a series of dead times in the control chain. If an LC element with an inductor in the longitudinal arm and a capacitor in the shunt arm is arranged in the output circuit and if--due to the aforesaid minimum pulse width--the positive voltage-time area applied to the output inductor during switching on is greater than the negative one in the switch-off phase, the magnetic energy rises and thus the current rises in an unlimited fashion from period to period. This is also confirmed by the following inequality which applies in the case of a terminal short circuit: EQU U.sub.E .multidot.E.multidot.t.sub.Emin &gt;U.sub.DF .multidot.(T-t.sub.Emin)(1)
where
U.sub.E =Input voltage PA1 u=Transformation ratio of the transformer PA1 t.sub.E min =Minimum switch-on period PA1 U.sub.DF =Conductive-state voltage of the freewheeling diode PA1 T=Duration of period. In FIG. 2, the typical operating states are illustrated: PA1 a=Onset of current limiting PA1 b=Threshold case in which the minimum switch-on period still just permits the limiting effect. PA1 c=Reduced limiting effect in the short circuit due to the condition (1)
As FIG. 2 shows, the problem described cannot be solved with pure instantaneous value limiting.
It is therefore expedient to provide an additional circuit for current limiting in the case of a short circuit.
The invention relates to a direct-current converter with current limiting.
A current converter of this kind is already known from DE-B1-28 38 009.
In the case of the known converter, a power switching transistor is driven by a clock generator with switch-on pulses at a constant operation frequency. In order to control the output voltage, the pulse duty cycle, that is to say the quotient of the switch-on time of the power switching transistor divided by the duration of the period, is changed. In the case of overloading at the output of the direct-current converter, the current in the power circuit is limited in order to protect the components. This is achieved by shortening the switch-on time. Here, the switching transistor is inhibited with the aid of a current measuring circuit as soon as the instantaneous value of the current flowing through the switching transistor, which current is measured with the aid of a current transformer, exceeds a predetermined limiting value.
Since the storage time of the switching transistor requires a minimum switch-on time and this minimum switch-on time would require an economically unacceptable over-dimensioning of the power circuit components in the case of a short circuit, in the known converter a further current measuring circuit, which is connected via a diode to the load of the current transformer, intervenes when the minimum switch-on time of the switching transistor ought to have been undershot and the voltage at the load of the current transformer has therefore risen by more than the threshold voltage of the diode. It interposes--if appropriate, repeatedly--switching pauses lasting for several periods so that the current in the power circuit does not rise above a predetermined limit. In this way, a larger degree of current limiting can be achieved.
The additional circuit ensures that the output inductor remains in magnetic equilibrium and the limiting effect is also maintained in the short circuit, but is associated with very high expenditure.